1. Field of the Invention
The present invention relates generally to devices for measuring the speed of fluid flow using pressure differential, and more specifically to devices for measuring the speed of watercraft.
2. Description of the Related Art
A commonly used device for the measurement of the air speed of airplanes or water speed of vessels is the Pitot tube. Typically, a Pitot tube is oriented facing the direction of travel through the fluid (water or air), and is positioned at a distance from the surface of the vessel, so that the Pitot tube is outside the boundary layer of the flow and therefore experiences the full flow. The pressure in the Pitot tube increases as the speed of the fluid passing by the opening of the tube increases. The pressure of the Pitot tube is measured or compared differentially to the pressure in a static tube oriented perpendicularly to the direction of fluid flow to determine the speed.
A related device used in the measurement of fluid flow is the Stanton tube. The Stanton tube is typically mounted on the surface over which the fluid is flowing, and therefore is in the boundary layer. The pressure in the Stanton tube is less than would be observed for a Pitot tube outside the boundary layer, and the Stanton tube must therefore be calibrated for the particular surface where it is mounted. Stanton tubes are generally not used as speedometers for aircraft or water vessels.
While the use of Pitot tubes as speedometers or flow meters is common, Pitot tubes have several drawbacks, especially when used in marine applications. Because they are generally mounted sticking away from the surface of a vessel, Pitot tubes are subject to damage from bumping. Collision with or snagging of flotsam or submerged debris, for example, driftwood, marine animals, or seaweed, can disable a Pitot tube. Airplane Pitot tubes can be damaged while on the ground, or by bird strikes, etc., in the air.
Similarly, Pitot tubes are subject to fouling by particles entering the tube. Bugs, marine life, sediment, etc., may all potentially enter the Pitot tube leading to inaccurate readings or complete failure of the device.
In addition, Pitot tubes inherently impose drag on the vessel to which they are attached. While this drag is not usually a problem for ordinary airplanes or boats, it may well pose a problem is in high performance applications.
The following examples of the conventional art illustrate various speed measuring devices based on sensing fluid pressure. U.S. Pat. No. 3,978,725, to Hadtke, entitled Speedometer Particularly For Water Skis, describes a water ski with a Pitot-tube like device disposed on the underside of the ski. A flexible diaphragm transmits the pressure from a tube opening to a fluid inside the ski, and thereby to a speedometer on the ski. The Pitot-tube like device has the tube opening on a portion of a rib on the underside of the ski.
U.S. Pat. No. 4,448,069, to Gibert, entitled Airspeed Sensing Post For Determining Relative Velocity Of A Fluid And A Carrier, discloses an airspeed sensing post with a static pressure sensing device of a particular shape, such that the measurement of static pressure is not affected by pitch angle. This device uses a standard Pitot tube, and does not solve the problems of Pitot tubes addressed above.
U.S. Pat. No. 4,920,808, to Sommer, entitled Device And Method For Measuring The Flow Velocity Of A Free Flow In Three Dimensions, describes a rotationally symmetrical flow body with at least two sets of peripherally spaced openings in regions of different thickness along the side of the body. The velocity of flow along the body is determined by differences in the static pressure measured in the openings. This device takes the form of a symmetrical flow body probe which presumably is mounted on a vehicle such as an aircraft or rocket in a position to experience the full flow, that is, away from the body of the vehicle. Thus, this device does not solve many of the problems with Pitot tubes described above.
U.S. Pat. No. 5,412,984, to Okita, entitled Vessel Speed Measuring System For The Marine Propulsion Machine, describes a vessel speedometer for a marine propulsion system such as an outboard motor. The design incorporates a Pitot tube pressure intake port in the lower leading edge of the outboard motor. Various designs to prevent fouling of the intake port, including ridges and projections around the intake port, are illustrated. This apparatus is specifically designed for use on a propulsion apparatus, however, and the apparatus requires that the intake port directly face the flow of the water.
U.S. Pat. No. 5,515,735, to Sarihan, entitled Micromachined Flow Sensor Device Using A Pressure Difference And Method of Manufacturing The Same, describes a micromachined flow sensor using a pressure differential. This device is related to Venturi-type devices, and such a device must be placed directly in the fluid flow.
U.S. Pat. No. 5,583,289, to Wiggerman et al., entitled Marine Velocity Detection Device With Channel To Wash Out Debris, describes a marine speedometer in which the pressure sensing orifice is protected by the curvature of a top portion of a tunnel through which the water flows. The overall apparatus must be placed in the direct fluid flow, and is mounted to a transom of the watercraft. This detection device is therefore a relatively bulky appendage to the watercraft.
Based on my reading of the art, I believe that what is needed is a flow meter or speedometer which may be mounted flush on the surface of a vessel so as not to protrude from the vessel as the standard Pitot tube does. Although non-protruding flow meters which directly face the direction of flow are available, as in U.S. Pat. No. 5,583,289, it is often impractical or undesirable to mount a flow detector on the nose of a vessel, that is, on the surface normal to the flow. Thus, I believe that what is needed is a flow meter which moreover may be mounted on a side surface over which the fluid is flowing roughly parallel to the surface.